Spent (contaminated) sulfuric acid is generated in various chemical production processes such as gasoline alkylation, titanium dioxide production, methyl methacrylate production, and various nitration processes to name but a few. Spent sulfuric acid from these processes has been disposed of by either deep well injection, or neutralization and discharge of the spent sulfuric acid into water ways, oceans or landfills. An accepted alternative to disposal has been the regeneration of spent sulfuric acid by first dissociating the sulfuric acid to obtain sulphur dioxide and other products. The sulfur dioxide is then oxidized to sulfur trioxide and the sulfur trioxide hydrated to form reusable sulfuric acid. The dissociation reaction is endothermic and normally carried out in a furnace. In other words, sulfuric acid dissociation requires the addition of heat since it has no intrinsic heating value. The energy for dissociation is provided by the combustion of residual hydrocarbons in the spent acid with air and supplemental fuels which can be either natural gas or fuel oil to sustain combustion. Combustion temperatures are between 1800.degree. and 2000.degree. F. resulting in flue gases containing 10% to 15% sulfur Dioxide along with oxygen, nitrogen, carbon dioxide and steam.
Regeneration of spent sulfuric acid is two to three times as expensive as acid made directly made from sulfur. The disposal of spent sulfuric acid is an ever increasing problem because of environmental regulations which are becoming more and more stringent. At the same time, demand for alkylates in unleaded gasoline is increasing thus, creating more spent sulfuric acid.
In an attempt to regenerate increasing amounts of spent sulfuric acid, oxygen enrichment of combustion air has been used to increase the capacity of a given regeneration facility. Use of oxygen enriched air permits more acid to be processed in an existing facility thereby improving the process economics to a certain degree. The combustion that is normally carried out with air, which contains 21% oxygen with the remainder being nitrogen, puts nitrogen into the process which plays no useful role in the waste combustion but leads to heat losses in the stack and reacts with oxygen to produce nitrogen oxides (known as thermal NO.sub.x), which in turn leads to smog formation, ozone depletion in the atmosphere and acid rain.
The formation of thermal NO.sub.x is extremely temperature sensitive. By enriching the combustion air to approximately 28% oxygen, the number of oxygen molecules available for combustion can be increased by 25% without increasing the volume of combustion air or flue gas. Hence, the waste processing capacity of a furnace can be increased. However, this approach has not been widely adopted in the market place because; oxygen enrichment leads to an increase in the furnace flame temperature including localized hot spots which have a detrimental effect on the materials of construction of the combustor and oxygen constitutes an additional cost of production that the regeneration facility has to incur. The extra cost for the oxygen is partially offset by the increased amount of acid processed in a given facility.
U.K. Patent 1,288,851 discloses and claims a process for regenerating sulfuric acid from spent acid, which patentee refers to as sludge, utilizing oxygen additions to air atomize spent sulfuric acid in a treatment furnace.
Other processes for treating spent sulfuric acid are disclosed in a published PCT application W093/13362 U.K. Patents 1,602,621 and 1,092,171; Canadian Patent Application 2,090,594; European Patent 0 091 679 and U.S. Pat. Nos. 4,256,721, 4,376,107; 5,022,332 and 5,266,297.
Another problem facing industry is the disposal of dewatered biosolids. Attempts at disposal of dewatered biosolids include the injection of dewatered biosolids into a municipal solid waste incineration system such as shown and described in U.S. Pat. No. 5,405,537. The process of the '537 patent uses the municipal solid waste as a heat source with the biosolids injected into the furnace for incineration.